A liquid crystal display is a display apparatus for displaying images and the like by applying voltage to control the orientation of the liquid crystals and passing or blocking light in each area. A back light provided on the back of a liquid crystal display is used as a light source for the liquid crystal display. Although a cold cathode tube has conventionally been used as the back light, a LED (light emitting diode) has recently been used in place of the cold cathode tube for the reasons of long operating life, bright colors, and the like.
Incidentally, nano-sized fluorescent substances using quantum dots have been commercialized in recent years. The quantum dots are luminescent semiconductor nanoparticles and the diameter range of the semiconductor nanoparticles is about 1 to 20 nm. Since the quantum dots show a wide excitation spectrum and have a high quantum efficiency, they are used as the fluorescent substance for converting an LED wavelength. Further, there is such a benefit that the emission wavelength of quantum dots can be completely controlled throughout the entire visible range only by changing the dot size of the quantum dots or the kind of a semiconductor material. For this reason, quantum dots are considered to have a potential of creating many different colors in practice, particularly warm white colors strongly desired in the lighting industry. Additionally, when three types of quantum dots corresponding to the emission wavelengths of red, green, and blue are combined, white lights with different color rendering indices can be obtained. As described above, the liquid crystal display using the back light lit by the quantum dots can improve the color tones and express many of the colors identifiable by a person without increasing thickness, power consumption, costs, and production process as compared with conventional ones.
The back light using a white color LED has a structure in which a fluorescent substance having a predetermined emission spectrum (quantum dots, or the like) is diffused in a film, the surface thereof is sealed with a barrier film, and in some cases a wavelength conversion sheet with a sealed edge portion is combined with an LED light source and a light guiding panel. The barrier film prevents a moisture and a gas from passing therethrough by forming a thin film by vapor deposition or the like on the surface of a substrate such as a plastic film. The barrier film is required to prevent poor appearance such as scratches and wrinkles and to be transparent in addition to have barrier properties. For example, Patent Literature 1 proposes a back light having a structure where a fluorescent substance is interposed between barrier films for preventing deterioration of the fluorescent substance.